In the fabrication of modem integrated circuit devices, one of the key requirements is the ability to construct plugs or interconnects in reduced dimensions such that they may be used in a multi-level metalization structure. The numerous processing steps involved require the formation of via holes for the plug or interconnect in a dimension of 0.5 .mu.m or less for use in high-density logic devices. For instance, in forming tungsten plugs by a chemical vapor deposition method, via holes in such small dimensions must be formed by etching through layers of oxide and spin-on-glass materials at a high etch rate. A high-density plasma etching process utilizing a fluorine chemistry is frequently used for such via formation process.
The via hole formation process can be enhanced by improving the etch directionality by a mechanism known as sidewall passivation to improve the anisotropy of the etching process. By utilizing a suitable etchant gas and reactor parameters, an etch-inhibiting film, normally of a polymeric nature, can be formed on vertical sidewalls. The etch-inhibiting film or the polymeric film slows down or completely stops any possible lateral etching of horizontal surfaces in the via hole. For instance, when a fluorine-containing etchant gas such as CFH.sub.3 is used, a fluorine-type polymeric film is formed on the sidewalls. Many photoresist materials may also contribute to the formation of a polymeric film on the sidewalls. After the sidewall is coated with a polymeric film, it is protected by the inhibitor film to preserve the line width or via hole diameter control.
In a modem etch chamber, an electrostatic wafer holding device, i.e., an electrostatic chuck or E-chuck, is frequently used in which the chuck electrostatically attracts and holds a wafer that is positioned on top. The E-chuck holding method is highly desirable in the vacuum handling and processing of wafers. In contrast to a conventional method of holding wafers by mechanical clamping means where only slow movement is allowed during wafer handling, an E-chuck device can hold and move wafers with a force equivalent to several tens of Torr pressure. Another advantage for the E-chuck arrangement is that no particle generation or contamination problem can occur since there are no moving parts acting on the wafer. Moreover, the electrostatic force utilized on an E-chuck is sufficient to prevent bowing of a wafer which normally occurs in mechanical clamping and thus promotes uniform heat transfer over the entire wafer surface.
In an etch chamber equipped with a plasma generating device and an electrostatic chuck for holding a wafer, a shadow ring is utilized as a seal around the peripheral edge of the wafer. The shadow ring, sometimes known as a focus ring, is utilized for achieving a more uniform plasma distribution over the entire surface of the wafer and to help restrict the distribution of the plasma cloud to stay only on the wafer surface area. The uniform distribution function is further enhanced by a RF bias voltage applied on the wafer during a plasma etching process. Another function served by the shadow ring is sealing at the wafer level the upper compartment of the etch chamber which contains the plasma from the lower compartment of the etch chamber which contains various mechanical components for controlling the E-chuck. This is an important function since it prevents the plasma from attacking the hardware components contained in the lower compartment of the etch chamber. In order to survive the high temperature and the hostile environment, the shadow ring is frequently constructed of a ceramic material such as quartz.
In a method for forming tungsten plugs by CVD tungsten deposition, via holes through insulating layers deposited on a pre-processed silicon wafer must first be opened. For instance, when tungsten plugs are formed on logic devices, via holes through composite layers of plasma enhanced oxide/spin-on-glass/plasma enhanced oxide must first be provided. In forming via openings in an etch chamber equipped with a quartz shadow ring, a plug loss defect has frequently been observed on the wafer particularly at areas close to the peripheral edge of the wafer. The plug loss defect is caused by lateral etch in a via opening such that a desirable diameter of the via opening cannot be maintained. The cause of the problem has been diagnosed as an interaction between oxygen gas generated by the quartz shadow ring when the ring is attacked by a fluorine-containing plasma and the polymeric passivation film coated in the via holes. The oxygen gas generated by the quartz shadow ring attacks the polymeric passivation film such that the passivation film is lost and thus subjecting the spin-on-glass (SOG) layer exposed in the via hole to lateral etching by the fluorine-containing plasma. The over etching of the SOG layer in the horizontal direction leads to the plug loss defect which prevents the formation of tungsten plugs in the desirable dimension that was intended.
It is therefore an object of the present invention to provide a shadow ring for use in an etch chamber that does not have the drawbacks or shortcomings of the conventional shadow rings.
It is another object of the present invention to provide a shadow ring for use in an etch chamber that is made of a material which does not generate contaminating oxygen gas when attacked by a fluorine-containing plasma.
It is a further object of the present invention to provide a composite shadow ring for use in an etch chamber wherein the ring has a surface that can not be attacked by a fluorine-containing plasma and thus any possible release of oxygen gas to destroy polymeric passivation layers in via opening can be prevented.
It is another further object of the present invention to provide a composite shadow ring for use in an etch chamber that comprises a body portion and an insert portion which intimately joins the body portion and is made of a material not subjected to attack by a fluorine-containing plasma.
It is still another object of the present invention to provide a composite shadow ring for use in a fluorine-containing plasma environment such that the portion of the shadow ring that is immediately adjacent to a wafer surface is made of a material which is inert to the fluorine-containing plasma generated in the etch chamber.
It is yet another object of the present invention to provide a composite shadow ring for use in an etch chamber that comprises a body portion made of a quartz material and an insert portion intimately joined to the body portion and is situated immediately adjacent to a wafer surface made of a material that does not generate oxygen gas upon attack by a fluorine-containing plasma.
It is still another further object of the present invention to provide a vacuum chamber for etching an electronic substrate that is equipped with a shadow ring constructed of two different materials wherein the material that is immediately adjacent to the wafer surface does not produce oxygen gas when attacked by a fluorine-containing plasma.
It is yet another further object of the present invention to provide a method for etching an electronic substrate in an etch chamber by utilizing a shadow ring that is constructed of at least one material which does not generate oxygen gas when attacked by a fluorine-containing plasma when the at least one material is positioned immediately adjacent to a wafer surface.